1. Field of the Invention
This invention relates to a resistance type temperature control system, and, in particular, to a system utilizing one or more condition responsive elements as part of an impedance network adapted to control the temperature in a variety of installations.
2. Prior Art
Apparatus and systems operating on resistance thermometer principles are well known and documented in the prior art. In particular, this invention constitutes an improvement over U.S. Pat. Nos. 2,362,977 and 2,431,790, issued on the joint inventions of David N. Crosthwait, Jr. and Everett W. Werts, on Nov. 21, 1944 and Dec. 2, 1947, respectively. Additionally, this invention represents an improvement over U.S. Pat. No. 2,260,135 to Werts and a later patent, U.S. Pat. No. 3,054,562, issued on Sept. 18, 1962. All of these prior art patents have been assigned to Dunham-Bush, Co., the assignee of this invention. Those prior art patents were directed to a variety of systems which were utilized to control the temperature in buildings, which utilized low pressure steam as the basic heating medium.
In the prior art, it has been recognized that systems operating on resistance thermometer principles are exceptionally sensitive and respond to minute electrical imbalances. Consequently, control systems operating on electrical bridge principles required skilled instrument technicians to insure continuity and stability of operation. Generally, however, for successful application to building temperature control, in situations where diverse skills and talents among those operating the system are common, such systems must be fabricated, installed, operated and maintained by persons who are not thoroughly familiar with the intricate operating techniques of the systems. In the prior art patents cited above, the control systems provide a construction and arrangement wherein a balanced resistance or resistance thermometer type control system is so constructed and arranged that it may be installed and operated by unskilled personnel. Despite the success achieved by those systems, areas of improvement exist.
For example, the efficient operation of a heating system requires that the operation of the temperature control equipment be accessible to and under the supervision of heating system operators. However, certain condition responsive elements must be located at remote points where they are under the influence of environmental conditions to be regulated. Also, portions of the system which require manipulation or adjustment during the operation of the system are located at a different location and, preferably, in the boiler room or at some central point in the building, while the condition responsive devices are located in each discrete compartment or other spaces where the temperature is to be controlled.
Given this wide diversity in location, the prior art has sought to establish effective control through a number of stabilization and compensation techniques.
In the basic system patent, U.S. Pat. No. 2,326,977, a temperature sensitive resistance thermometer is mounted on the inside surface of a pane of window glass in the environmental area to be controlled. The controller circuitry reacts to the inner surface temperature of the glass as a measure of the heat requirements of that area as determined by prevailing outdoor conditions, inside air temperature and mean radiant temperature of interior parts of the area. Through the use of a pair of temperature sensitive resistances, one mounted below and the other above the heating element in a convector type of heating unit, the controller reacts to the rise in temperature of the air stream passing through this heating element as a measure of the rate of heat output from the heating system. Additionally, in the Crosthwait system, a mechanically operated potentiometer is utilized to adjust a motor which operates the control valve in the system such that this potentiometer is in synchronism with the particular valve. Hence, the reactor is responsive to the degree of opening of the control valve, and by combinations of the ennumerated sensing circuits in Wheatstone bridge circuits, a variety of automatic controls are achieved.
In an improvement of the Crosthwait system, U.S. Pat. No. 2,620,135 attempted to overcome the problem of distance in a typical operating condition wherein lengthy conductive paths were required between distantly located control responsive elements and the centrally located control panel. As pointed out in the specification of that patent, in typical operations, condition responsive elements could be placed 5,000 feet or more from the control panel, and in a situation where a number of leads were required, the length of wire would become inordinately large. Also, the problem of sensitivity in the Wheatstone bridge configurations was the subject of the improvement in the Werts (U.S. Pat. No. 2,620,135) application. As pointed out, the bridge circuitry is very sensitive to small electrical imbalances, and residual resistance due to corrosion, loose contact, etc. at the point of contact between the sliding arm of the potentiometer and the resistor portion of the potentiometer will upset the bridge balance when the potentiometer is in a bridge arm. To eliminate this problem, the Werts invention eliminates the use of potentiometers in the bridge arms themselves so that there is no possibility of unbalance occurring because of varying resistances in the sliding context of the potentiometers. The remote arms of the bridge are interconnected, and, accordingly, the effects of lead compensation are obtained with fewer connecting wires from remote arms to the other branches than in the prior art Crosthwait arrangement. Since the conductors in the circuit act as junctions of the bridge branches, they preserve the effects of lead compensation while at the same time eliminating the necessity of providing certain separate compensating leads.
A similar improvement, found in U.S. Pat. No. 2,891,218, dealt with utilizing the remotely located elements, but substituting alternating current for the prior utilized direct current source. As is well known, a bridge circuit utilizing alternating current is inoperative unless the capacitance factor is compensated, and the Werts U.S. patent (No. 2,891,218) makes this compensation possible by the location of the resistors 24 adjacent to the resistance thermometer 21 and connecting it with comparable leads 25 and 26 in the arm 18 of the bridge, as shown in FIG. 1 of U.S. Pat. No. 2,891,218. With an arrangement as shown in FIG. 1, leads 25 and 26 provide not only the necessary resistance balance, but also the capacitance between the leads and between the leads and the ground, which would then be the same as in the arm 17 of the FIG. 1 bridge. Hence, the bridge circuitry of the U.S. Pat. No. 2,891,218 utilizing alternating current finds itself as an improvement in the state of the art since amplication techniques available in AC circuits are thereby made available for utilization in the temperature control circuit, which would not be previously utilized if direct current were the power source in the system.